(1) Field of the Invention
The present invention relates to a technique for monitoring an optical signal to noise ratio in an optical transmission system. In particular, the invention relates to a method of monitoring an optical signal to noise ratio based on the degree of polarization of an optical signal, and an optical transmission system using the same.
(2) Description of Related Art
Recently, optical wavelength division multiplexing transmission systems of 10 Gb/s (gigabits/second) have started to come into practical use. However, due to the recent rapid increase in network usage, the further increase of capacity and the ultra long distance of network are required, and research and development of optical wavelength division multiplexing transmission systems with transmission speeds of 40 Gb/s or higher per wavelength have been carried out actively inside and outside of Japan. In order to realize such an ultra long distance optical transmission system of 40 Gb/s or higher, it is important to ensure an optical signal to noise ratio (referred to hereunder as optical SNR) and to improve stability thereof. Specifically, in optical transmission systems with a transmission speed per wavelength of 40 Gb/s or higher, since an optical SNR is deteriorated theoretically compared with optical transmission systems of 10 Gb/s, such that for example, the optical SNR at 40 Gb/s transmission is deteriorated by 6 dB relative to an optical SNR at 10 Gb/s transmission, it is essential to ensure a high optical SNR in order to realize the ultra long distance optical transmission.
In general, in order to ensure a required optical SNR in the whole of an optical transmission system, the introduction of various techniques of, for example, setting an optical output power of a transmission terminal (that is, optical input power to an optical transmission path) to be higher in a range in which waveform deterioration due to a self phase modulation (SPM) effect does not occur, adequately considering the configuration of the transmission terminal, or applying Raman amplifiers to repeater nodes, are effective, and implementation of these techniques in optical transmission systems of 40 Gb/s or higher has been investigated actively.
However, even if the techniques as described above are introduced, since there is no margin for the required optical SNR in the optical transmission system of 40 Gb/s or higher, a technique for monitoring an optical SNR is important for ensuring a high optical SNR and maintaining stability thereof, at the time of operating the system. A spectrum analyzer unit (SAU) or the like is used, for example, as means for monitoring the optical SNR in a conventional optical transmission system.
Incidentally, one factor limiting a transmission distance in the optical transmission system of 40 Gb/s or higher is polarization-mode dispersion (to be referred to hereunder as PMD). PMD is a phenomenon wherein a group delay difference is caused between two cross polarization mode components since a core shape of transmission fiber is slightly elliptical, thus causing waveform deterioration. To be specific, in old fibers installed in mainly the overseas, there are some fibers having a large PMD value exceeding 0.5 to 2 ps/√km per unit length, and the transmission distance at 40 Gb/s optical transmission is limited to approximately 3 to 50 km if the worst PMD value is assumed to be three times an average.
In such a transmission path environment, in order to realize long distance transmission of optical signal at 40 Gb/s or higher, since the generated PMD is changed with time according to a change in the transmission path environment, such as temperature, stress and the like, it is necessary to monitor the polarization of the optical signal while the system is operating, to compensate for the PMD dynamically, and hence a PMD compensation technique has been proposed to deal with it (refer to Japanese Unexamined Patent Publication No. 2002-16548).
However in many cases, for the purposes of miniaturization and low cost, the spectrum analyzer unit used to monitor the optical SNR in the optical transmission system as described above, has only a function targeted to detect the power of optical signal, but has a problem in measurement accuracy at 40 Gb/s or higher optical transmission, which requires the optical SNR to be monitored with high accuracy. Furthermore, considering dynamic PMD compensation, it is necessary to provide a unit to monitor the polarization of optical signal in addition to the above-described spectrum analyzer unit, which will make the system configuration complicated and expensive.